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Unread 11-12-2014, 04:55 PM   #1
willyoung
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Newbie Deflection and load bearing joist calculations

Hello All,

I’ll get the backstory out of the way firstly…

I’m currently remodelling an early 1900s terrace house, and was a little concerned as to the strength of the floor joists above the main living room. -

I ran some deflect calculations for the existing joists and deflection wise it was way below modern building regs of L/360…

The max Unsupported Joist span for the room is: 14ft (4.25m-4.3m depending on partition wall brickwork)

Joist width: 3 inch (true)
Joist Height: 7 inch (true).

Joist condition: Most joists are in fairly good condition, although several have been somewhat butchered by tradesmen over the years, one of the joists in the dead centre of the room has a 25mm hole bang on centre of the joist…and two 19mm holes about 2 foot away either side of this 25mm hole (for radiator pipes).

So i ran the existing joists through the deflecto at L/279 if i state the wood is in good condition, although I’m a little skeptical due to several of the joists having had notches and one of them having several holes within the middle third of the joist.

Being able to feel the bounce when waling across the floor, i went about some strengthening measures….

First off, i put in two rows of solid blocking spaced at thirds for all joists - blocking was some 6x2 i had spare, glued and screwed with 100mm “10’s” screws…this seemed to take a little of the bounce out of the floor although i still felt it could be made much better..

By this point, i’ve already got the downstairs ceilings ripped off and we are partway through the first fix re-wiring and plumbing so sistering any joists didn’t seem like a viable option, plus it seemed a bit excessive to sister every joist - especially as finding true 3x7’s is pretty hard these days (i didn’t think of just sistering 8x2s or 8x10s plus didn’t want cost to start rocketing…)…so i opted for the method of adding material to the bottom of the joist, as some on the forum have previously discussed…this seemed a cheaper option, and wouldn’t cause any disruption to the wiring nor pipework i’d already put in place…

On every joist i glued and screwed (screws at maximum every 10 inches) 4x2’s (true size 1.5 x 3.5) to make an inverted T shape.

After taking this measure the bounce in the floor was noticeably reduced and, dare i say, probably would just about meet modern regs for deflection with an approximate deflecto reading of L/398 - this based on having wood in “good” condition, 3 inched wide, 8 inches high…I figured its best to err on side of caution since glue and screw method probably wouldn’t swell as a single wooden piece…of course this assumes my inverted t shape works about as good as standard 3x8.


So what I’ve already done has made a big improvement. Except i’ve missed off some information…

The joist thats dead centre in the room (the one with a couple of small holes in the middle third) supports a stud wall, and this stud wall helps support a drop ceiling i’ve put in for the upstairs bedroom.

This drop ceiling consists of 6x2’s on small spans, but above the 6x2s is the old lathe a plaster ceiling which, due to sagging the 6x2s are taking some of the weight of (as to get the 6x2’s level, even with a small gap it meant pushing the old lathe and plaster ceiling up a bit.)

Now in the attic, i’ve boarded out approximately 50% of the floor space, so there will be a small amount of light storage weight being transferred to the drop ceiling and stud wall respectively which will end up being supported partially by our main downstairs ceiling joist…

as for the actual amount of weight that will be transferred to our joist in question from the attic storage, i don’t think it will be all that much as the attic is primarily supported by a series of adjacent stud walls (that support their own drop ceilings, that reside over the main internal load bearing wall from the ground floor..so the vast majority of load will be transferred via this.

(i hope everyone is still following…i may have to make some diagrams to visually explain things a bit better)..

So our main joist in question is already taking some dead weight from beefed up stud wall, drop ceiling and a small amount possibly from attic storage.

The floor feels decent to walk on after my alterations and i doubt anythings ever going to come down, as the current amount of dead weight on our joists shouldn’t be enough to cause any problems, nor even long term creep…


BUT heres my concern and what i need some advice on..

Due to the house layout, and pipework, i’d like to install radiators back to back for the bedrooms at either sides of the stud wall (that sits on main joist- parallel onto of the joist)…now these radiators would probably amount to about 50kg together maximum(110lbs roughly) and the smallest radiator is about 3.5foot long, but both would be sitting over the central third of the joist…

My worry is not really over any more bounce in the floor (as it feels pretty good at the moment), but more to do with long term creep of the main joist with the weight of the radiators being permanently fixed onto the stud wall…

I’ve gone through the calculations for a distributed loads of 100lbs / foot along the joist, and if i con sider my reinforced joists as being 3x8’s then we meet the required section modulus (if I’ve got calcs right…we’d need modulus of 29.4 cubic inch and an 8x3 should give 32.3 cubic inch so i think the strength of it should be ok…please someone correct me if I’ve got that wrong)

However, when doing a point load calculation (which i think is more relevant in this scenario) im getting bit stuck on the deflection calculation and the example I’m using…from here: totalconstructionhelp.com/beams.html has me baffled….i can’t seem to get the example to work :S

So i was wondering if anyone could help a newbie out here and give me an idea of whether my existing joists could handle the radiators mounted on them, or alternatively what can i do to make things strong enough to support them.

Since i haven’t wired anything into the mains consumer unit yet, it IS possible to get wires out of any joists and sister wood to our critical joist (would mean removing blocking and a general pain but IS possible).

Alternatively i could move one of the radiators in on of the rooms so its sitting on a different wall that sits across joists very close to the downstairs load bearing brick wall…although i would prefer to keep the rads back to back and they’re much better positioned in the room…but obviously if its going to involve lots of time and money then i’ll just have to live with things..


Apologies for the really long post - i just wanted to try paint as fuller picture as possible, and give as much information as possible….i’m open to suggestions and can get more info if needed plus pictures and can make some diagrams too with measure,nets on them!!

Thanks,
Will
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Unread 11-12-2014, 05:20 PM   #2
cx
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Welcome, Will.

Yes, it was very long. But nowhere in there do I see any mention of the species, grade or spacing of these joists. I also see no mention of subflooring at all.

But perhaps the biggest unanswered question is, why? Why are you doing all this floor structure work? Is there perhaps a ceramic or stone tile installation involved somewhere in this project that we should know about?

I see the term "main joist" a number of times. Early on I thought this was just another joist near the center of the structure, but then it sounded more like it was some sort of support beam. Is it a different thing from your floor joists?
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Unread 11-15-2014, 02:46 AM   #3
willyoung
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Hi CX,

Thanks for replying - re-reading my first post it really is an essay!

Joist spacing is 16 inch OC.

Sub Flooring is 22mm tongue and groove floorboards - wood type i think could be pine

As for the species and grading of the joists themselves im not sure, although the species of wood does appear to be a lot stronger than whatever species of wood is most commonly used in modern joists these days. Its certainly a lot more difficult to drill through! The house is an early 1900s terrace, and being in the UK im not sure what kind of wood they would have had available at that time, nor how it might typically compare to what species you might commonly use in the US...although if i were to guess id say it should compare to southern yellow pine or douglas fir. maybe a cross section picture of a joist might help identify species better?


The original reason for wanting to improve the floor was that the old plasterboard ceiling in the ground floor living room had noticeable cracks in the plaster where the plasterboard sheets met. I had suspected this could be due to excessive deflection in the first floor joists above. Running the floor specs through the deflecto, it gave deflection of 0.6inch (L/279 - unknown wood good condition, 7 inch tall, 3 inch wide, 16 inch OC, 14 foot span)

Since i was doing a total remodel and modernisation of the house, i didnt like the idea of my freshly plastered and painted ceilings becoming cracked after a short period of time and having to be constantly patched up over the years.

Apologies for the confusion over the term "main joist" - you were right at first in thinking its just a floor joist near the center of the structure. I suppose, i've been referring to it more as if it were a beam, due to their being a stud wall above it.

Hopefully the attached pictures might give a better understanding of how things currently are. I can get more pictures if there are any specifics anyone would like to see - e.g. cross section of end of a joist!

So really the main question of: "will the existing structure support some radiators (approx 50kg/110lbs) mounted onto the stud wall that sits of the joist in the center of the structure, without excessive long term creep?"
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Unread 11-15-2014, 10:06 AM   #4
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Will, if you'll add a geographic location to your User Profile it would be permanently displayed and perhaps help folks understand some of the terminology and dimensions are just the result of us being from two nations divided by a common language rather than any attempt at obfuscation on your part, eh?

And I still see no indication that there is to be a ceramic tile installation involved here. Is that the case?

The initial structural design is not very sound, but as for being able to hang a hundred-pound radiator on the wall, all I can say is "would you allow an eight-stone person to stand in that spot?" Or two such persons? There is not likely to be any danger of a structural failure even if you invited several friends to stand together on each side of the wall, and even less chance with just a small radiator attached to the wall.

My opinion; worth price charged.
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